In Morrowind they cost 1000 gold per slave. In reality it would depend entirely on the skills / race / gender of the slave in particular. Argonians are the most common - often used for hazardous tasks / harsh life-shortening labour in mines or fields due to them being easily replaceable as the Dres territory borders Black Marsh.

An educated Altmer/Imperial slave would likely be more expensive, but naturally not as many would be needed. In the Summerset Isle goblins are enslaved - which given their limited intelligence I suspect would be incredibly cheap. Some races such as Orcs can be difficult to keep due to their temperments.

It depends on the Mage/Wizard - the Telvanni use slaves for their Kwama Mines - they can be worked near to death, don't need to be paid & only need bare basic feeding/shelter. Further the slave once it has 'expired' could become research material - depending on the mage & their scholarly interests.

Although it is worth noting slavery across borders / in Imperial waters is illegal in the Empire - many resorting to illegal slave smuggling to cater to the market.

As for a 'free man/mer/betmer' commoners typically can work for bare pittance - shelter, provisions & bare bones salary - much alike how the peasants lived in the real world. Although again the higher quality/skilled persons would cost more - it may be easier to train a slave.